1. Field of the Invention
This invention relates to barrier type photovoltaic cells useful for converting light and particularly solar energy into electrical energy.
2. State of the Prior Art
So-called Schottky barrier photocells rely upon the fact that certain metals, i.e., those having an appropriate work function, form a rectifying contact when appropriately contacted with a suitable semiconductor material. Electron-hole pairs generated by the absorption of light in the semiconductor are separated due to the field at the metal-semiconductor junction, establishing an electrical potential. In common with most of the early photocells, however, such cells had low conversion efficiencies due to the rather low open-circuit voltages that were produced. It is well-known that the conversion efficiency is a direct mathematical function of the open-circuit voltage.
Early attempts to manufacture barrier-type photovoltaic cells included devices formed by vacuum depositing metal onto cadmium telluride. Cadmium telluride has long been known to be an optimum material for photocells because its band gap is closest to that which will accommodate the optimum amount of solar energy. As explained in General Electric Technical Report AFAPL-TR69-32, gold was one of the metals thoroughly studied. However, this initial work concluded that preheating the cadmium telluride in oxygen pursuant to the methods of the prior art gave no enhancement of the photocell properties.
More recent developments have raised a question about the earlier conclusion that preheating the semiconductor yields an ineffective enhancement, at least with regard to gallium arsenide as the semiconductor material. It was reported in Applied Physics Letters, Vol. 27, July 1975, pages 95-97, that the open-circuit voltage of a gold-coated GaAs cell was improved about 200 mV by heating the semiconductor in air up to 200.degree. C. for 4 to 70 hours prior to coating with gold. However, a subsequent paper by the same authors, entitled "The AMOS Cell--An Improved Metal-Semiconductor Solar Cell," presented at the Eleventh Photovoltaic Specialist Conference at Scottsdale, Ariz. on May 6-8, 1975, reported that the results were not reproducible no matter what temperature or what length of time were used, particularly when a different source of oxygen, such as pure oxygen, was used. Instead, an additional undisclosed element was said to be required for maximum enhancement.
Although some success may have been achieved by preheating GaAs, the art has been discouraged from attempting to preheat other semiconductor materials by publications such as "Improved Schottky Barrier Solar Cells" published by the authors noted above at the aforesaid Eleventh Photovoltaic Specialists Conference, page 391. It is well-known that the open-circuit voltage (V.sub.OC) of a photovoltaic cell can be expressed as the equation ##EQU1## where n=the diode factor
k=the Boltzmann constant PA0 T=the absolute temperature PA0 q=the electronic charge PA0 I.sub.L =the light-generated current PA0 A*=the modified Richardson's constant, and PA0 .phi..sub.B =equilibrium barrier height.
It can be shown that, for an untreated n-type GaAs cell coated with gold, the predicted untreated value is about 500 mV, which compares favorably with actual experimental values. However, it can also be shown, using equation (1), that the calculated value for untreated n-type CdTe coated with gold is only about 200 mV so that, even if an increase of 200 mV as was achieved in GaAs devices could be achieved for CdTe by preheating, the resultant 400 mV would not reach the untreated value for GaAs.
Silicon barrier solar cells have been formed wherein the silicon surface was oxidized prior to coating with the barrier metal, either deliberately or by reason for example of the heating step used to fuse the ohmic metal contact to the opposite surface. Such devices resulted in an enhanced V.sub.OC value, as is shown, for example, in J. of Applied Physics, Vol. 46, No. 9, page 3982, Sept. 1975.
The general background of photovoltaic cells, and/or to the use of common features such as grid electrodes, antireflection layers, etc., is described in U.S. Pat. Nos. 3,888,697; 3,703,408, 3,769,558; and 3,811,954. Articles pertaining to such general background include "A New Look at CdTe Solar Cells", Bell and Wald, Eleventh Photovoltaic Specialist Conference, supra at page 497; and "Recent Research on Photovoltaic Solar Energy Converters", Loferski, 1963 Proceedings of the IEEE, page 667.